instmctions): The central hypotheses of Research Project II are 1) that reading and writing are complex skills that depend on a number of component processes with both shared and independent genetic and environmental etiologies in children with disabilities, and 2) that pattems of covariation in etiology will provide unique insights into the cognitive architecture underlying these skills and into the nature of learning disabilities. Project II will assess component processes in reading, writing, and related phonological skills in twins and siblings selected for deficits in reading, writing, and/or ADHD, and in normal-range control twins. Our long- running assessment of word recognition, orthographic coding, phonological decoding, and phonological awareness in Project II will be continued to support more powerful and infomnative behavioral and molecular genetic analyses of these skills, but our main emphasis w\\\ be on the relations of these skills to writing and to comprehension (Project V), and on writing's relations to processing speed, executive functions and ADHD assessed in Projects I and III. New measures of writing have been added to this project to allow the first thorough analysis of genetic and environmental influences on its component processes, with expanded assessment of compositional writing. Oral composition is also assessed to separate effects of composition from modality of response. The broad age range in the proposed sample will facilitate cross-sectional analyses of important developmental differences in writing and its relations to discourse skills assessed by reading comprehension, oral language, executive functions, processing speed, and ADHD. Univariate multiple regression (DF) analyses will be used to evaluate the genetic and environmental etiologies for deficits in each skill. Bivariate DF analyses and confirmatory factor analyses of twin data will be used to assess the genetic and environmental covariance among the different skills. Behavior-genetic analyses will also be used to validate subtypes and/or dimensions of individual differences among children with reading and writing disabilities by assessing their differential genetic and environmental etiology and gene x environment interactions. In collaboration with Research Project IV, QTL analyses will be used to assess genetic linkage and association for deficits in the reading, writing, and phonological skills assessed in Project II. RELEVANCE (See instructions): Reading and writing are critically important skills for academic and professional developmenL Leaming disabilities in these skills are important public health problems in need of better understanding of their classification and their genetic and environmental etiologies. The proposed behavior- and molecular-genetic studies on reading, writing, and related skills assessed in Project II will contribute to these goals.